How to select all objects in a scene?
To select every object in the scene press A. This makes to select / deselect everything depending on whether something was already selected. So if there's something already selected press A twice.

Note that there can be hidden objects in the scene, which won't be selected without unhiding them. You can do that by pressing Alt+H to unhide and then proceeding to select. See hiding / unhiding everything but selected objects; also toggling visibility of object in Outliner.

If you'd like to select everything in scene depending on some category then you should use Select Grouped menu which can be accessed by Shift+G when in Object mode. To select only meshes select one, access menu and choose Type entry. Same with any other type of objects.



See more about selecting in selection tools wiki